Question

How would the potential energy of the car change

Answer 1

if the car is running and suddenly got down a break then its potential energy change from the p.e.of car when it was off

Answer 2

(i) The potential energy of the car remains same, since PE (= mgh) is independent of velocity.
(ii) The kinetic energy of the car becomes four times, since KE(=1/2 m ${{v}^{2}}$) j is proportional to square of velocity.
(iii) The momentum of the car will also get doubled, since momentum (p = mv) is proportional to velocity.